For many years, traditional wrenches have been widely used. Traditional wrenches are frequently found inconvenient, inefficient, cumbersome and new wrenches are highly demanded. For instance, wrenches having closed openings with planar engagement surfaces often result in stress being applied to a hexagonal fastener and thus cause deformation of the fastener as well as providing inefficient torquing ability. Prior art wrenches having closed openings with curved engagement surfaces are known. See U.S. Pat. No. 4,930,378 to Colvin. Although the prior art wrenches include six-point and twelve-point versions, problems with stress distribution are not resolved. Furthermore, the twelve-point version is configured using either metric or inch sizing, rather than both as in a duplex system. Ruzicka et al. discloses rotary wrenching tools capable of wrenching fastener heads in various measuring systems (U.S. Pat. No. 5,388,486. to Ruzicka et al.), but does not disclose a duplex system for both metric and inch sizes. Heberlie discloses ornamental designs for a double-ended box wrench showing the contour of the wrench heads. U.S. Pat. No. D303,069. The invention, however, discloses a double-ended one-sided crown. As the DIYers encounter an increasing number of metric fastener and diminishing percentage of inch sizes, they are required to have two sets of wrenches, for both inch and metric sizes, which are cumbersome, weighty and costly in investment.